1. Field of the Invention
The present invention relates to a circuit for supplying power to an apparatus which uses a battery as a power source.
2. Description of the Prior Art
The voltage of a power source which comprises a battery has constraints.
In FIG. 1, a reference numeral 1 denotes a battery; 2 a low voltage circuit system; 3 a voltage step up circuit; 4 a high voltage circuit system. In the thus formed power source, the circuits which are operated at a battery voltage are directly driven with a battery, and the other circuits are driven at a necessary voltage which is stepped up by the step up circuit.
The electric power which is consumed by the circuit of the high voltage circuit system is represented as the product of the stepped up voltage, the conducted electric current and the inverse of the efficiency of the step up circuit. The electric power which is consumed by a low voltage circuit system is represented as the product of the battery voltage and the conducted current. Since the life time of the battery depends upon the power consumed by the high voltage circuit system 4 in FIG. 1, efforts have been made to reduce the conducted current and to enhance the efficiency of the step-up circuit 3. However, there have been constraints in these efforts.
An approach to protect a power source when the voltage of a battery is momentarily lowered is disclosed in Japanese Patent Application (JP-A-2-51335, published Feb. 21, 1990) entitled "BATTERY POWER SOURCE EQUIPMENT". This apparatus does not necessarily aim at reducing power consumption and has different merits.